The use of light scattering from particles is well known and has led to such extensively used systems as the Laser Doppler Anemometer (LDA) as taught in Reynolds et al., U.S. Pat. No. 4,715,707 and, more recently, Particle Imaging Velocimetry (PIV) (see Adrian, R. J., "Int. J. of Heat and Fluid Flow", Vol. 7, page 127 (1986). In these cases, the motion of particles is tracked either using coherent processes or rapid sequential images. Resolving individual particles over a wide field of view requires exceedingly high resolution detection, so LDA and PIV are generally useful only where the field-of-view is limited and the particle seeding concentration is carefully controlled. In air, particle concentrations may vary over a wide region and, in particular, at high altitudes, particle concentrations may be very low and particle sizes so small that scattering from single particles becomes an unreliable method for measuring airspeed. Similarly, in high speed flow facilities, large particles may not track the flow, and velocities may need to be measured in an instantaneous fashion. In the latter case, randomly occurring particles may not be present at the proper location when the measurement is made.
While the present invention may be used in conjunction with single particles, its major strength is that velocities can be measured by observing scattering from multiple particles or direct scattering from air molecules. Molecular scattering, which is called Rayleigh scattering, has recently been shown to be a powerful new tool for optical flow field diagnostics [see B. Yip, D. Fourguette, and M. B. Long, Applied Optics, Vol. 25, page 3919 (1986)]. In particular, Rayleigh scattering using ultraviolet light [see M. Smith, A. Smits, and R. Miles, Opt. Lett., Vol. 14, page 916 (1989)]has produced two-dimensional cross-sectional images of high-speed flow fields. Similar images of scattering from very high densities of small particles have been used to observe cross sections of combusting gases and mixing phenomena. One of the embodiments of this invention is configured in a manner similar to Rayleigh scattering or many particle imaging systems, but with an absorption line filter window to allow the determination of the velocity component field. Alternate approaches to the measurement of velocity based on nonlinear optical phenomena such as stimulated Raman gain spectroscopy (see Exton, U.S. Pat. No. 4,624,561) integrate the velocity along a line and cannot be used for imaging. They are also double-ended, requiring a retroreflector or detector at the far end of the sample cell. This means they cannot be used for ranging and are of limited application in flow facilities.
Optical systems in aircraft which have been used to observe airspeed and to look ahead for wind shear rely on particle scattering. The normal particle scattering systems involve collecting the scattered light returned and performing optical heterodyne (see Breen, U.S. Pat. No. 4,822,164), interferometric (see German DT 2500376 and Domey et al, U.S. Pat. No. 4,334,779), or spectral analysis (see Woodfield, U.S. Pat. No. 4,585,341) of that scattered light as a method to compare it with the original laser beam that was sent. Spectral analysis, interferomic, and optical heterodyne techniques are subject to noise fluctuations due to the random phases of the scattered light. Such detection techniques are only accurate when the scattered light appears to be a point source, so that random phase interferences across the face of the optical detector or interferometer do not wash-out the signal. As a consequence, such detector schemes cannot be used to observe volume scattering phenomena or to image. At high altitudes, particle scattering is significantly reduced so these techniques become unreliable.
Another embodiment of the invention is a device to accurately measure frequency shifts associated with either volumetric multiparticle scattering or Rayleigh scattering so aircraft airspeed and wind shear can be detected. This device will operate even in the absence of particles and can be used in a LIDAR (Light Detection and Ranging) type configuration to give velocity as a function of distance from the aircraft with a single laser pulse. This makes the detection of velocity discontinuities such as those associated with clear turbulence more easily detectable.
The use of atomic resonant line filters to detect narrow linewidth sources has been described by Gelbwachs [see J. A. Gelbwachs, IEEE J. Quant. Electronics, Vol. 24, page 1268 (1988)]and others. In their devices, a narrow linewidth source overlaps specific atomic or molecular transitions, leading to fluorescence or photoionization. These atomic filters do not rely on the coherence of the light and, therefore, can be used to observe single frequency light coming from all angles. Generally, the efficiency of these fluorescence or ionization type filters is poor (approximately 2%), but their selectivity is very high since off-resonance light produces virtually zero background. They are intrinsically non-imaging since the original light is totally absorbed, but some configurations using very strong absorption and cellular design are able to give low resolution imaging capability [see E. Korevaar, M. Rivers, C. S. Liu, SPIE Vol. 1059, Space Sensinq Communications and Networking, p. 111 (1989)]. For the applications discussed here, it is important to have a filter with a high efficiency since light scattered by small particles and Rayleigh scattering are extremely weak. Atomic blocking filters have been proposed by She et al. for LIDAR applications [see H. Shimizu, S. A. Lee, C. Y. She, Appl. Opt., Vol. 22, p. 1373 (1983)]. In this case, the narrow absorption line is used to remove particle scattering so temperature and density measurements can be made with Rayleigh signals.
The use of atomic or molecular filters has been described by Komine (U.S. Pat No. 4,919,536) for the measurement of velocity fields by laser light scattering from particles. His approach requires that the flow be seeded with particles, and that the filter cutoff be a linear function of frequency.